A VP-SEM, which uses gas multiplication, is an electron beam apparatus that detects secondary electrons emitted from a sample into a low vacuum atmosphere by irradiating primary electron beams to form image and is different from a general high vacuum SEM in view of the detection principle and the apparatus configuration.
Secondary electrons emitted from a sample by irradiating primary electron beams are accelerated due to an electric field and ionize the remaining gas molecules repeatedly. The electrons and ions are amplified (gas multiplication) and the amplified ions/electrons are detected by a detection electrode, thereby forming images. The ions/electrons are detected as a positive/negative displacement current that flows in the detection electrode. In order to move the generated ions/electrons to the detection electrode, any potential gradient is set between the detection electrode and a place where the ions/electrons are generate.
In order to describe a concept of the displacement current, FIG. 15 shows an equivalent circuit of the detector.
The equivalent circuit of the detector is represented by a condenser 901. Herein, it is considered when a capacitance C to which voltage V is applied and a charge q 902 within a condenser space having a distance L between the electrodes. The charge q moves in a direction perpendicular to the electrode by electric field V/L within the condenser space. At this time, if a fine movement amount is dx, the potential between the electrodes is changed by d(=(V/L)·q·dx. In order to compensate for the changed amount in the potential, the charge dQ is induced in the condenser. The equilibrium equation of the potential is given as d(=V·dQ and its result is dQ=(q/L)·dx. Since I=dQ/dt, I=(q/L)·v. This is a displacement current 903. Herein, v=dx/dt and is a velocity of the charge q.
Actually, the ions and electrons are mixed in the condenser by gas multiplication. At this time, the electrons move to a high potential electrode and the ions move to a low potential electrode, such that the displacement current flows in the same direction.
There are two detection methods, one is a method 904 that connects an electrode of a low voltage side, which is the detection electrode, to an amplification circuit to detect a positive displacement current and the other a method 905 that connects an electrode of a high voltage side, which is the detection electrode, to the amplification circuit to detect a negative displacement current. Although a physical principle of both methods is completely the same, the potential gradient supplied into the sample chamber or the shape of the electrode (electric field supplying electrode) for supplying the electric field, are different. Further, there are various detectors depending on the intended used.
Herein, it is noted that the charge moving in a vertical direction (vertical to an x-axis of FIG. 15) with respect to the electric field of the condenser is not detected as the displacement current. Further, it is noted that the response of the detector is determined based on the movement time of the ions whose movement is delayed. Generally, the movement time of ions is longer than the response time of the amplification circuit.
The inventors suggest scanning electron microscopes with the detector according to the method of detecting the positive displacement current. For example, JP-A-2001-126655 discloses a VP-SEM having a form that arranges a detection electrode on a sample holder and uses a secondary electron collector electrode (connected to the high-vacuum secondary electron detector) as an electric field supplying electrode arranged around an objective lens. The condenser space is formed between the electric field supplying electrode and the sample holder. Further, JP-A-2003-132830 discloses the VP-SEM having a form that can increase detection efficiency of current by disposing a detection electrode having a curved shape between an electric field supplying electrode and a sample holder (on a path generating the ionization scattering). Moreover, JP-A-2006-228586 discloses a configuration of a detection electrode enclosing the circumference of an electric field supplying electrode so as to shorten the movement time of the ions and increase the detection efficiency. In JP-A-2003-132830 and JP-A-2006-228586, the condenser space is formed between the electric field electrode and the detection electrode.
As the method of detecting the negative displacement current, there is G. D. Danilatos et al., Scanning 3, 215 (1980). The method uses both an electron current detecting electrode and an electric field supplying electrode, in which the potential of a surface of the detection electrode is maintained to be higher than that of the circumference of the detection electrode. The condenser space is formed between the electric field supplying electrode (detection electrode) and the sample holder. Further, JP-T-Hei9(1997)-501010 of Electro Scan Corporation suggests a detector that includes an electrode reducing a noise signal by the method of detecting the negative displacement current and as one embodiment, disclose an embodiment forming a configuration, which includes an electric field supplying electrode in a ring shape and an electrode that reduces a signal from a back scattered electron source or a noise signal from a primary electron beam source, on a print board. In this configuration, the condenser space is formed between the electric field supplying electrode (detection electrode) and the sample holder.